Sound image localizing processor

ABSTRACT

A sound image localization processor comprises a first processing circuit  10  receiving a sound left signal and comprising a first delay unit  11  and a first sound image localization filter  12 , a second processing circuit  20  receiving a surround right signal and comprising a second delay unit  21  and a second sound image localization filter  22 , an adder  1  for adding the surround left signal and an output signal of the second processing circuit  20  and outputting the result of the addition as a voice signal to a left loudspeaker located ahead of a listener, and an adder  2  for adding the surround right signal and an output signal of the first processing circuit  10  and outputting the result of the addition as a voice signal to a right loudspeaker located ahead of the listener.

TECHNICAL FIELD

The present invention relates to a sound image localization processorfor making a listener feel without using a surround loudspeaker as if asurround signal of a two-channel stereo were outputted from the surroundloudspeaker using two loudspeakers located ahead of the listener.

BACKGROUND ART

FIG. 6 illustrates a conventional sound image localization processingcircuit.

A surround left signal SL inputted to an input terminal P1 is fed to afirst sound image localization filter 101 and a second sound imagelocalization filter 102. In each of the filters 101 and 102, filterprocessing corresponding to a filter coefficient of the filter isperformed.

A surround right signal SR inputted to an input terminal P2 is fed to athird sound image localization filter 103 and a fourth sound imagelocalization filter 104. In each of the filters 103 and 104, filterprocessing corresponding to a filter coefficient of the filter isperformed. The characteristics of the first sound image localizationfilter 101 and the characteristics of the fourth sound imagelocalization filter 104 are the same, and the characteristics of thesecond sound image localization filter 102 and the characteristics ofthe third sound image localization filter 103 are the same.

An output of the first sound image localization filter 101 and an outputof the third sound image localization filter 103 are added together inan adder 111, and the result of the addition is outputted as L_(OUT).The output L_(OUT) is fed to a left loudspeaker located at the left andahead of a listener.

An output of the second sound image localization filter 102 and anoutput of the fourth sound image localization filter 104 are addedtogether in an adder 112, and the result of the addition is outputted asR_(OUT). The output R_(OUT) is fed to a right loudspeaker located at theright and ahead of the listener.

Each of the sound image localization filters is found by a headtransmission function, described below. Generally used as the soundimage localization filter is an FIR (Finite Impulse Response) digitalfilter having several hundred taps.

Description is made of a method of calculating a sound imagelocalization filter using a head transmission function.

As shown in FIG. 7, let H_(LL), H_(LR), H_(RL), and H_(RR) berespectively transmission functions for each transmission path from realloudspeakers L and R arranged at the left and right and ahead of alistener 100 to the left and right ears of the listener 100. Let W_(L)and W_(R) be respectively transmission functions from a virtual soundsource position P where a sound is desired to be localized to the leftand right ears of the listener 100. All the transmission functions aredescribed on the frequency axis.

In order that a voice can be heard by the listener 100 as if it wereoutputted from the virtual sound source position irrespective of thefact that the voice is outputted from the real loudspeakers L and R, thefollowing equation (1) must hold, letting X be an input signal, andletting L_(OUT) and R_(OUT) be respectively output signals from the realloudspeakers L and R. $\begin{matrix}{{\begin{pmatrix}W_{L} \\W_{R}\end{pmatrix}\quad X} = {\begin{pmatrix}H_{LL} & H_{LR} \\H_{RL} & H_{RR}\end{pmatrix}\quad \begin{pmatrix}L_{OUT} \\R_{OUT}\end{pmatrix}}} & (1)\end{matrix}$

Consequently, the signals L_(OUT) and R_(OUT) respectively outputtedfrom the real loudspeakers L and R are found by the following equation(2): $\begin{matrix}{\begin{pmatrix}L_{OUT} \\R_{OUT}\end{pmatrix} = {\frac{1}{{H_{LL}\quad H_{RR}} - {H_{LR}\quad H_{RL}}}\quad \begin{pmatrix}H_{RR} & {- H_{LR}} \\{- H_{RL}} & H_{LL}\end{pmatrix}\quad \begin{pmatrix}W_{L} \\W_{R}\end{pmatrix}\quad X}} & (2)\end{matrix}$

Furthermore, if it is assumed that the real loudspeakers L and R arelocated so as to be bilaterally symmetrical , as viewed from thelistener 100, the transmission functions which are bilaterallysymmetrical are the same. Accordingly, the following equations (3) and(4) hold. The same transmission functions are respectively taken asH_(THR) and H_(CRS).

H _(THR) =H _(LL) =H _(RR)  (3)

H _(CRS) =H _(LR) =H _(RL)  (4)

Consequently, the foregoing equation (2) can be rewritten to thefollowing equation (5): $\begin{matrix}\begin{matrix}{\begin{pmatrix}L_{OUT} \\R_{OUT}\end{pmatrix} = {\frac{1}{{H_{LL}\quad H_{RR}} - {H_{LR}\quad H_{RL}}}\quad \begin{pmatrix}H_{RR} & {- H_{LR}} \\{- H_{RL}} & H_{LL}\end{pmatrix}\quad \begin{pmatrix}W_{L} \\W_{R}\end{pmatrix}\quad X}} \\{= {\frac{1}{H_{THR}^{2} - H_{CRS}^{2}}\quad \begin{pmatrix}H_{THR} & {- H_{CRS}} \\{- H_{CRS}} & H_{THR}\end{pmatrix}\quad \begin{pmatrix}W_{L} \\W_{R}\end{pmatrix}\quad X}} \\{= {\begin{pmatrix}\frac{{H_{THR}\quad W_{L}} - {H_{CRS}\quad W_{R}}}{H_{THR}^{2} - H_{CRS}^{2}} \\\frac{{H_{THR}\quad W_{R}} - {H_{CRS}\quad W_{L}}}{H_{THR}^{2} - H_{CRS}^{2}}\end{pmatrix}\quad X}} \\{= {\left( \frac{H_{1}}{H_{2}} \right)\quad X}} \\{\begin{pmatrix}{H_{1} = \frac{{H_{THR}\quad W_{L}} - {H_{CRS}\quad W_{R}}}{H_{THR}^{2} - H_{CRS}^{2}}} \\{H_{2} = \frac{{H_{THR}\quad W_{R}} - {H_{CRS}\quad W_{L}}}{H_{THR}^{2} - H_{CRS}^{2}}}\end{pmatrix}}\end{matrix} & (5)\end{matrix}$

As a filter in which H₁ and H₂ in the equation (5) are converted intotime axes, an FIR digital filter having several hundred taps is used.

The frequency characteristics of the first sound image localizationfilter 101 and the fourth sound image localization filter 104 shown inFIG. 6 correspond to H₁ in the equation 5, and the frequencycharacteristics of the second sound image localization filter 102 andthe third sound image localization filter 103 correspond to H₂ in theequation 5.

The FIR digital filter is generally realized by a digital processor suchas DSP (Digital Signal Processor). When the DSP, for example, is usedfor this processing, the number of processing steps required therefor isapproximately the same as the number of taps of the FIR digital filter.As the overall amount of processing, therefore, processing whose amountis four times the number of taps of the FIR digital filter is requiredbecause there are four FIR digital filters.

Specifically, 1000 or more processing steps are required for the digitalsignal processor. Further, the FIR digital filter found by such acalculating method generally has complicated frequency characteristics.Therefore, a signal which has been subjected to FIR digital filterprocessing reasonably has a sharp peak dip, so that it becomes a soundwhich is unnatural and has an uncomfortable feeling. An example of thefrequency characteristics of the FIR digital filter used for sound imagelocalization is shown in FIG. 8.

FIG. 9 illustrates a circuit for reproducing a multi-channel audiosignal such as DolbyDigital or MPEG only on two channels utilizing thesound image localization processing technique shown in FIG. 6. In FIG.9, the same portions as those shown in FIG. 6 are assigned the samereference numerals.

A left signal L and a right signal are added to a signal obtained bysubjecting a center signal C to gain control of −3 dB by a multiplier121, respectively, by an adder 113 and an adder 114.

An output of the adder 113 and the output of the adder 111 described inFIG. 6 are added together by an adder 115, and the result of theaddition is taken as an output L_(OUT) to a left loudspeaker. An outputof the adder 114 and the output of the adder 112 described in FIG. 6 areadded together by an adder 116, and the result of the addition is takenas an output R_(OUT) to a right loudspeaker.

Also in such a circuit, much of the processing is processing of the FIRdigital filter for sound image localization of a surround signal, sothat a large burden is imposed on the DSP. Further, the FIR digitalfilter found by the head transmission function is used. Accordingly, thetone becomes unnatural.

An object of the present invention is to provide a sound imagelocalization processor corresponding to a surround signal, in which theamount of processing can be reduced and a more natural tone is obtained.

DISCLOSURE OF INVENTION

In a sound image localization processor for making a listener feelwithout using a surround loudspeaker as if a surround signal of atwo-channel stereo were outputted from the surround loudspeaker usingright and left two loudspeakers which are located ahead of the listener,a first sound image localization processor according to the presentinvention is characterized by comprising a first processing circuitreceiving a surround left signal and comprising a first delay unit and afirst sound image localization filter; a second processing circuitreceiving a surround right signal and comprising a second delay unit anda second sound image localization filter; an adder for adding thesurround left signal and an output signal of the second processingcircuit and outputting the result of the addition as a voice signal tothe left loudspeaker located ahead of the listener; and an adder foradding the surround right signal and an output signal of the firstprocessing circuit and outputting the result of the addition as a voicesignal to the right loudspeaker located ahead of the listener.

In a sound image localization processor for making a listener feelwithout using a surround loudspeaker as if a surround signal of atwo-channel stereo were outputted from the surround loudspeaker usingright and left two loudspeakers which are located ahead of the listener,a second sound image localization processor according to the presentinvention is characterized by comprising a first low-pass filterreceiving a surround left signal; a second low-pass filter receiving asurround right signal; a first processing circuit receiving an outputsignal of the first low-pass filter and comprising a first delay unitand a first sound image localization filter; a second processing circuitreceiving an output signal of a second low-pass filter and comprising asecond delay unit and a second sound image localization filter; an adderfor adding the output signal of the first low-pass filter and an outputsignal of the second processing circuit and outputting the result of theaddition as a voice signal to the left loudspeaker located ahead of thelistener; and an adder for adding the output signal of the secondlow-pass filter and an output signal of the first processing circuit andoutputting the result of the addition as a voice signal to the rightloudspeaker located ahead of the listener.

A digital delay unit may be used as each of the delay units, and each ofthe sound image localization filters may be constituted by a pluralityof IIR digital filters. An analog delay unit may be used as each of thedelay units, and each of the sound image localization filters may beconstituted by a plurality of IIR digital filters.

A digital delay unit may be used as each of the delay units, and each ofthe sound image localization filters may be constituted by a pluralityof analog filters. An analog delay unit may be used as each of the delayunits, and each of the sound image localization filters may beconstituted by a plurality of analog filters.

As a low-pass filter, a digital low-pass filter may be used, or ananalog low-pass filter may be used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a sound image localizationprocessing circuit according to a first embodiment of the presentinvention;

FIG. 2 is a graph showing an example of the characteristics of asecondary IIR digital filter in a case where a series connection of twosecondary digital filters is used as a sound image localization filter;

FIG. 3 is a circuit diagram showing a circuit for reproducing amulti-channel audio signal such as DolbyDigital or MPEG only on twochannels utilizing a sound image localization processing technique shownin FIG. 1;

FIG. 4 is a circuit diagram showing a sound image localizationprocessing circuit according to a second embodiment of the presentinvention;

FIG. 5 is a circuit diagram showing a circuit for reproducing amulti-channel audio signal such as DolbyDigital or MPEG only on twochannels utilizing a sound image localization processing technique shownin FIG. 4;

FIG. 6 is a circuit diagram showing a conventional sound imagelocalization processing circuit;

FIG. 7 is a schematic view for explaining a method of calculating asound image localization filter using a head transmission function;

FIG. 8 is a graph showing an example of the frequency characteristics ofan FIR digital filter used for the sound image localization processingcircuit shown in FIG. 6; and

FIG. 9 is a circuit diagram showing a circuit for reproducing amulti-channel audio signal such as DolbyDigital or MPEG only on twochannels utilizing a sound image localization processing technique shownin FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1 to 5, embodiments of the present invention willbe described.

[1] Description of First Embodiment

FIG. 1 illustrates the configuration of a sound image localizationprocessing circuit.

A surround left signal SL inputted to an input terminal P1 is fed to afirst adder 1 as well as to a first processing circuit 10 comprising adelay unit 11 and a sound image localization filter 12.

A surround right signal SR inputted to an input terminal P2 is fed to asecond adder 2 as well as a second processing circuit 20 comprising adelay unit 21 and a sound image localization filter 22.

In the first adder 1, the surround left signal SL and an output signalof the second processing circuit 20 are added together. An output signalLOUT of the first adder 1 is fed to a left loudspeaker located at theleft and ahead of a listener.

In the second adder 2, the surround right signal SR and an output signalof the first processing circuit 10 are added together. An output signalR_(OUT) of the second adder 2 is fed to a right loudspeaker located atthe right and ahead of the listener.

As the delay units 11 and 21, either one of a digital delay unit and ananalog delay unit may be used. The sound image localization filter 12and the sound image localization filter 22 have the samecharacteristics. As the sound image localization filters 12 and 22, acombination of one to five low order IIR (Infinite Impulse Response)digital filters or a combination of one to five analog filters havingthe same characteristics as those of the IIR digital filter may be used.

In the present embodiment, the digital delay unit is used as the delayunits 11 and 21. A hearing experiment proves that 3 to 15 sampling timeperiods are preferable as the amount of delay. The 3 to 15 sampling timeperiods are selected in consideration of the respective characteristicsand listening positions of the delay units.

In the present embodiment, a series connection of two secondary IIRdigital filters is used as each of the sound image localization filters12 and 22. An example of the composite frequency characteristics of thesecondary IIR digital filter is illustrated in FIG. 2.

As a result, a surround signal can be felt as if it were outputted froma surround loudspeaker. Further, a more natural tone than that in theconventional example is obtained.

When an IIR digital filter or a combination of IIR digital filters isused as each of the sound image localization filters 12 and 22, it ispossible to arbitrarily select the characteristics of the IIR digitalfilter, the number of the IIR digital filters, the order of the IIRdigital filter, and a connecting method (in parallel or series) of theIIR digital filters can be arbitrarily selected.

Although in the above-mentioned embodiment, each of the processingcircuits 10 and 20 comprises a delay unit whose amount of delaycorresponds to 3 to 15 sampling time periods and a sound imagelocalization filter which is a combination of one to five low order IIRdigital filters. Accordingly, the amount of processing can be made muchsmaller, as compared with that in the conventional example using the FIRdigital filter. Further, in the low order IIR digital filter, smootherfrequency characteristics than that in the FIR digital filter can beobtained, so that a more natural tone is obtained.

FIG. 3 illustrates a circuit for reproducing a multi-channel audiosignal such as DolbyDigital or MPEG only on two channels utilizing thesound image localization processing technique shown in FIG. 1. In FIG.3, the same portions as those shown in FIG. 1 are assigned the samereference numerals.

A left signal L and a right signal R are added to a signal obtained bysubjecting a center signal C to gain control of −3 dB by a multiplier 7,respectively, by a third adder 3 and a fourth adder 4.

An output of the third adder 3 and the output of the first adder 1described in FIG. 1 are added together by a fifth adder 5, and theresult of the addition is taken as an output L_(OUT) to a leftloudspeaker. An output of the fourth adder 4 and the output of thesecond adder 2 described in FIG. 1 are added together by a sixth adder6, and the result of the addition is taken as an output R_(OUT) to aright loudspeaker.

In such a circuit, the amount of processing is reduced, and a morenatural tone is obtained, as in the circuit shown in FIG. 1.

[2] Description of Second Embodiment

FIG. 4 illustrates the configuration of a sound image localizationprocessing circuit. In FIG. 4, the same portions as those shown in FIG.1 are assigned the same reference numerals and hence, the descriptionthereof is not repeated.

In the circuit, a surround left signal SL inputted to an input terminalP1 is fed to a first adder 1 through a first low-pass filter 30 as wellas to a first processing circuit 10 comprising a delay unit 11 and asound image localization filter 12.

Similarly, a surround right signal SR inputted to an input terminal P2is fed to a second adder 2 through a second low-pass filter 40 as wellas to a second processing circuit 20 comprising a delay unit 21 and asound image localization filter 22.

Specifically, the circuit differs from the circuit shown in FIG. 1 inthat the low-pass filters 30 and 40 for relieving an uncomfortablefeeling in a high frequency band. As the low-pass filters 30 and 40, adigital low-pass filter may be used, or an analog low-pass filter may beused.

The first low-pass filter 30 comprises a multiplier 31 for subjectingthe input signal SL to gain control of −6 dB, a delay unit 32 fordelaying an output signal of the multiplier 31 by one sampling timeperiod, and an adder 33 for adding the output signal of the multiplier31 and an output signal of the delay unit 32 together in this example.

The second low-pass filter 40 comprises a multiplier 31 for subjectingthe input signal SR to gain control of −6 dB, a delay unit 42 fordelaying an output signal of the multiplier 41 by one sampling timeperiod, and an adder 43 for adding the output signal of the multiplier41 and an output signal of the delay unit 42 together in this example.

FIG. 5 illustrates a circuit for reproducing a multi-channel audiosignal such as DolbyDigital or MPEG only on two channels utilizing thesound image localization processing technique shown in FIG. 4. In FIG.5, the same portions as those shown in FIG. 4 are assigned the samereference numerals.

A left signal L and a right signal R are added to a signal obtained bysubjecting a center signal C to gain control of −3 dB, respectively, bya third adder 3 and a fourth adder 4.

An output of the third adder 3 and an output of the first adder 1 areadded together by a fifth adder 5, and the result of the addition istaken as an output L_(OUT) to a left loudspeaker. An output of thefourth adder 4 and an output of the second adder 2 are added together bya sixth adder 6, and the result of the addition is taken as an outputR_(OUT) to a right loudspeaker.

What is claimed is:
 1. In a sound image localization processor formaking a listener feel without using a surround loudspeaker as if asurround signal of a two-channel stereo were outputted from the surroundloudspeaker using right and left two loudspeakers which are locatedahead of the listener, the sound image localization processorcharacterized by comprising: a first processing circuit receiving asurround left signal and comprising a first delay device and a firstsound image localization filter; a second processing circuit receiving asurround right signal and comprising a second delay device and a secondsound image localization filter; an adder for adding the surround leftsignal and an output signal of the second processing circuit andoutputting the result of the addition as a voice signal to the leftloudspeaker located ahead of the listener; and an adder for adding thesurround right signal and an output signal of the first processingcircuit and outputting the result of the addition as a voice signal tothe right loudspeaker located ahead of the listener.
 2. In a sound imagelocalization processor for making a listener feel without using asurround loudspeaker as if a surround signal of a two-channel stereowere outputted from the surround loudspeaker using right and left twoloudspeakers which are located ahead of the listener, the sound imagelocalization processor characterized by comprising: a first low-passfilter receiving a surround left signal; a second low-pass filterreceiving a surround right signal; a first processing circuit receivingan output signal of the first low-pass filter and comprising a firstdelay device and a first sound image localization filter; a secondprocessing circuit receiving an output signal of a second low-passfilter and comprising a second delay device and a second sound imagelocalization filter; an adder for adding the output signal of the firstlow-pass filter and an output signal of the second processing circuitand outputting the result of the addition as a voice signal to the leftloudspeaker located ahead of the listener; and an adder for adding theoutput signal of the second low-pass filter and an output signal of thefirst processing circuit and outputting the result of the addition as avoice signal to the right loudspeaker located ahead of the listener. 3.The sound image localization processor according to either one of claims1 and 2, wherein each of the delay devices is a digital delay device,and each of the sound image localization filters is constituted by aplurality of IIR digital filters.
 4. The sound image localizationprocessor according to either one of claims 1 and 2, wherein each of thedelay devices is an analog delay device, and each of the sound imagelocalization filters is constituted by a plurality of IIR digitalfilters.
 5. The sound image localization processor according to eitherone of claims 1 and 2, wherein each of the delay devices is a digitaldelay device, and each of the sound image localization filters isconstituted by a plurality of analog filters.
 6. The sound imagelocalization processor according to either one of claims 1 and 2,wherein each of the delay devices is an analog delay device, and each ofthe sound image localization filters is constituted by a plurality ofanalog filters.